Is there any explanation in all this for "why" this was McD's policy? I'm guessing it was because they didn't want to deal with complaints about the coffee being cold, but that still stretches logic.

My (admittedly dim) recollection was that many (though not all) customers who purchased coffee from the drive-through would actually drink the coffee quite some time later - ie., when they were done driving. So McDonald's would heat the coffee well above 'normal' drinking temperature, so that it would be closer to that 'normal' drinking temperature when the customer finally drank it.